Air pollution control

ABSTRACT

COMBUSTION GASSES FROM FUEL-BURNING FURNACES ARE TREATED TO SEPARATE SOLID PARTICLES SUCH AS FLY ASH AND TO OXIDIZE THE SULFUR DIOXIDE GAS (SO2) CONTAINED THEREIN TO SULFUR TRIOXIDE (SO3). THE SO3 IS REACTED WITH EXCESS LIME (CAO) TO FORM CALCIUM SULFATE (CASO4). THE CASO4, UNSPENT LIME PARTICLES, SOME OF WHICH HAVE CRACKED OUTER SHELLS OF CASO4, AND COLLECTED FLY ASH ARE PACKAGED DRY AND USED IN APPROPRIATELY BLENDED FORM TOGETHER WITH CEMENT AND SAND, GRAVEL, OR ROCK TO FROM CONSTRUCTION MATERIALS, THE AVERAGE PERCENTAGE OF CASO4 THEREIN BEING REGULATED TO CONTROL THE SETTING TIME OF THE CEMENT. THE PROPORTION OF THE EXCESS LIME IS AUTOMATICALLY REGULATED BY PROCESS-CONTROL DEVICES WHICH CONTINUOUSLY MONITOR AND CONTROL THE REACTION APPLYING THE PARAMETERS OF FUEL COMPOSITIONS, AND SULFUR OXIDE LEVEL IN THE COMBUSTION GASSES.

Dec. 25, 1973 PING"WHA LIN 3,781,408

AIR POLLUTION CONTROL Filed March 16, 1972 2 Sheets-Sheet 1 F l G: 1 700MAGNIFICATION OF PARTICLE United States "Patent Otfice US. Cl. 423-24410 Claims ABSTRACT OF THE DISCLOSURE Combustion gasses from fuel-burningfurnaces are treated to separate solid particles such as fly ash and tooxidize the sulfur dioxide gas (S contained therein to sulfur trioxide(S0 The S0 is reacted with excess lime (CaO) to form calcium sulfate(CaSO The CaSOq. unspent lime particles, some of which have crackedouter shells of CaSO and collected fly ash are packaged dry and used inappropriately blended form together with cement and sand, gravel, orrock to form construction materials, the average percentage of CaSOtherein being regulated to control the setting time of the cement. Theproportion of the excess lime is automatically regulated byprocess-control devices which continuously monitor and control thereaction applying the parameters of fuel composition, and sulfur oxidelevel in the combustion gasses.

RELATED APPLICATION This application is a continuation-in-part ofcopending US. application Ser. No. 36,861 invented by Ping-Wha- Lin,filed May 13, 1970 and now abandoned.

BACKGROUND OF THE INVENTION The invention relates to air pollutioncontrol, and more particularly to pollution control of effluent gassesthat contain fly ash and sulfur-bearing gasses.

Air pollution is a very serious and urgent international problem. Thesources of air pollution are the products of combustion and are almostalways numerous and widespread. Alone, the United States is currentlypouring pollutants into the atmosphere at the staggering rate of 160million tons a year.

Much of the air pollutants are in the form of fly ash and sulfur-bearinggasses discharged by coal-burning electrical power generating plants orother industries, or even by oil-consuming heaters in private homes orother buildings. These pollutants actually cause untold millions orbillions of dollars of annual damage by corroding buildings andstructures, and also cause innumerable cases of human suffering fromvarious respiratory diseases plus the above-mentioned global effects.Yet, under any foreseeable circumstances, we will have to burn more andmore fuel to meet the demand of rapidly growing population requiring foreach person ever-more heating comfort and electrical power. And the fuelwe shall use will not contain much less, but likely contain more sulfurand fly ash.

This invention discloses methods and equipment designed not only toremove these pollutants but to convert them into useful and salableby-products, such as construction materials.

While not limited thereto, the invention is herein described as appliedto combustion gasses discharged into the air from a power plant andcontaining fly ash and sulfur-bearing gasses.

An important object of the invention is to control the pollution of airfrom effluent gasses containing sulfur oxide gas and solid particlessuch as fly ash;

Another object is to remove fly ash and sulfur oxide bearing gasses fromcombustion gasses in an economical manner;

3,781,408 Patented Dec. 25, 1973 Yet another object of this invention isto collect air pollutants and convert them into useful by-product forms;

A further object is to use the collected pollutants for specialtreatment and economical manufacture of cinder block or otherconstruction materials;

Another object is to provide means and methods for controlling thecomposition of the combustion gasses through automatic process controlof the pollutants-treating processes, based on monitored data relatingto the type and amount of pollutants in the combustion gasses, and theefficiency of the various treating processes, with due regard to thefuel quality and requirement of the manufactured construction materials.

A further object is to produce from the air pollutants usefulconstruction materials of novel characteristics.

An overall object of the invention is to provide a continuous processfor effectively removing sulfur dioxide as a pollutant from combustiongasses and to do so by means of substantially converting the sulfurdioxide and then exposing such sulfur trioxide to a fluidized bed ofcalcium oxide under such reaction conditions that virtually all of thesulfur trioxide is reacted to form calcium sulfate and is hence removedfrom the outflow of combustion product gasses.

Further objects and advantages of my invention will appear as thespecification proceeds.

SUMMARY OF THE INVENTION My invention discloses methods and equipmentsfor removing S0 S0 and fly ash in combustion gasses discharged as airpollutants from fossil fuel-burning, heating furnaces. The removal isaccomplished by means of dust collectors and catalytic oxidation of S0to S0 followed by controlled reaction of S0 with excess lime to formCaSO The unspent lime, 02180 and collected fly ash are used togetherwith cement for the manufacture of construction materials, thepercentage of CaSO being carefully regulated to control the setting timeof the cement. Preferably, the proportion of the excess lime isautomatically regulated by process control devices designed with dueregard to fuel quality and requirement of the construction materials andoperating according to the monitored sulfur-level of the combustiongasses.

BRIEF DESCRIPTION OF "II-IE DRAWINGS The preferred form of my inventionis illustrated in the accompanying drawings, in which:

FIG. 1 is a photomicrograph of 700x magnification of a calcium oxideparticle coated with calcium sulfate and produced as by-product of thepresent invention and DESCRIPTION OF THE PREFERRED EMBODIMENTS In theflow diagram of FIG. 3 is shown a furnace 10 in which fossil fuelcontaining sulfur and inert residues is burned with air, which ispreheated for better heating efficiency. The combustion gasses from thefurnace 10 contain S0 S0 and suspended solid particles consisting ofnoncombustible residue and partially burned fuel.

According to J. W. Mellow (in A Comprehensive Treatise on Inorganic andTheoretical Chemistry, vol. X,

Wiley, N.Y., 1961, pp. 90 to 91), sulfur burns in air to form mostly Sbut with a few percent by weight of 80;, in the oxidation products. Whenthe oxidation products are drawn over ferric oxide, such as is usuallypresent in coal, the quantity of S0 can be increased to 17%. If the aircontains moisture, a little sulfuric acid (H 804) is also formed. Thesetwo compounds are desirable to the practice of my invention, as will beshown.

The combustion gasses are passed through an air preheater 12 and areregulated to an elevated temperature, preferably to about 900 F. beforebeing passed through a mechanical dust collector 14 and an electrostaticprecipitator 16 system for the separation of solid particles in the formof fly ash. The dust collector system comprises a mechanical dustcollector 14 to remove large fly ash particles and a high temperatureelectrostatic precipitator 16 to separate the finer particles in thecombustion gasses. Mechanical dust collectors usually separate entrainedparticles in gasses by centrifugal force. Electrostatic precipitatorsmay be used for the removal of practically any material suspended ingasses by electrically attracting these particles.

The dust collector system should have nearly 100% eiiiciency insolid-particle removal, particularly with regard to those particleswhich may posion catalysts in the catalytic oxidation convertor to bedescribed. Separated fly ash particles are collected and sent to ablender or storage bin 18.

The flue gas, i.e., combustion gasses cleaned of the suspendedparticles, then passes through a sulfur-level monitor 20, whichdetermines the level of sulfur concentration in the flue gas, and has acomputerized controller 21 and a servo connection 22 to a lime supply 24connected by supply iine 25 to lime reactor 28. The flue gas containssulfur oxides SO, mainly as S0,, and S0 and is then passed through acatalytic oxidation converter 26. A typical catalytic convertor containsvanadium catalyst having a very large surface to volume ratio andoperated at elevated temperatures 650 F.-950 'F., preferably between 850F. to 880 F., to oxidize S0 into 80;, according to the followingreaction:

2 2 (gJ- s (a) It is evident that increasing the pressure of theconvertor, which is preferable in some cases, shifts the chemicalequilibrium toward the right thereby enhancing the efliciency of thedesired conversion of S0 to S0 However, according to R. F. Bovier (inProceedings of the American Power Conference, vol. 26, 1964), even atone atmospheric pressure, the conversion eificiency is typically over90%.

The flue gas coming into the lime reactor 28 generally contains around9% of moisture. The following reactions take place at above 880 F. inthe reactor:

CaO and CaSO, can absorb a large quantity of water at room temperature.At high temperatures, however, these compounds absorb very little or nowater. Hence, these compounds coming out of the lime reactor 28 can bemoisture-free, and can even remain so if immediately packaged in dryatmosphere.

The S0 may also combine with CaO directly to form CaSO,

CaO (s.)-l-SO (g.) CaSO (s.)

For reasons of increased agitation, better heat transfer, high reactionefliciency, small floor space, economy installation and operation, andother factors, the lime reactor 28 or reacting system should becontinuously operated and comprises one or more counter-current,multi-stage, fluidized CaO reactors.

Since the S0 concentration in the stack gas is quite low, the monitor 20is responsive to the S0,, content which 4 is essentially S0 While thefeed rate of lime is not high, its feed rate is correlated to the S0content by means of the computerized controller 21 which, by means ofservo line 22 regulates the correct feed rate of lime from lime supply24 through a rotary feeder in line 25 to lime reactor 28. Under usualreaction conditions approximately 35 mg. of pulverized lime issuflicient for each liter of flue gas for total sulfur oxide gasremoval.

The feed rate of flue gas as well as the sulfur oxide gas content isvariable but at all times the lime supply is correlated thereto. Thecontroller is programmed with due regard to the fuel quality andrequirements of the manufactured construction materials and by the servooperation to effect a near total removal of the S0 content of the fluegasses. A monitor 29 is located in the flue gas outlet line 30 from theline reactor 28 so that if for any reason the equipment is notperforming satisfactorily the operation will signal a malfunctioning toindicate the need for adjustment repair or shut down.

The exit solid stream from the lime reactor 28 consists of particles ofdifferent degrees of conversion from CaO to CaSO The average conversionof this stream depends mainly on two factors:

(A) The rate of reaction of lime particles in the reactor environment,and

(B) The residence time distribution of the solids in the lime reactor.

The chemical reaction between and CaO is highly exothermic. But sincethe concentration of S0,, in the flue gas is low (i.e., 2000 p.p.m. byvolume), the heat generated from the lime reaction can be made to justmeet the heating requirement for the cold, freshly-added lime.Therefore, it is expected that the reactor 28 will be operated at thesame temperature as the incoming flue gas, i.e. above 650 F. andpreferably above 850 F. The cold lime may be efliciently heated byspirallying down inside the lime reactor to extract some heat from thehot gasses discharged from the reactor.

The particle size of CaO is important in the fluidization process, as itaffects solids flow characteristics, bed densities, losses of solids,and equipment erosion. According to Othmer (see, e.g., Fluidization,Reinhold Publishing Corporation, N.Y., 1956), a broad range of particlesizes gives better fluidization than a narrow range; and fine,pulverized materials are better than coarse. Commercially available,ground quicklime, typically about 100% passing No. 8 sieve and 2 to 4%passing No. 100 sieve, can be used in the fluidized lime reactor,pulverized lime 100% passing No. 20 sieve, also %-95% passing sieve isavailable and is satisfactory. The governing factors, in the fluidizedlime reactor design are: densities of solids, particle sizes,concentration of S0 in the stack gas, and flow rate of stack gas.Knowing all these factors, skilled chemical engineers can select thereactor to meet individual design requirements.

The C210 size distribution is particularly important in the practice ofmy invention for another reason. CaO is fed purposely in excess of theamount required for complete conversion of S0, to CaSO Hence, some (3210particles are barely, or even not at all, changed at the surface orinterior. Other extremely fine CaO particles 32 (FIGS. 1, 2, 4, 5) may,however, be completely converted to CaSO But most CaO particles are onlypartially converted on their surfaces, giving cracked 13 coatings 15 ofCaSO, 34 on cores of unspent CaO 35 as shown in FIG. 4. CaO may beporous, like a sponge (FIG. 5). In this case, the surface of the pores37 are also converted into CaSO (FIG. 5). These new structures ofunspent lime achieve novel results that will be discussed.

The surface layers of CaSO, are only of the order of 15 microns (10-cm.) in thickness. This is because the residence time z at the limeparticles in the lime reactor is estimated to be in the range of 10 to100 minutes, depending on the particle size distribution, gas flow rate,fluidized reactor design, and other variables. The diffusion coeflicientof S0 through CaSO layers, D, to reach and react with the inner, unspentCaO can be considered to be about cmF/sec. at a temperature above 850 F.The penetration depth, or surface CaSO layer thickness, is

then about /Dt,. or in the range of 7.8 to 23.2 microns. These CaSOlayers contain cracks 13 because of the sudden cooling in quenchingchamber 46 from the preferred temperature above 850 F. to ambienttemperature.

The reacted lime particles are discharged from lime reactor 28 throughline 42 having a rotary feeder 44 and into a quenching chamber 46. Whenthe hot CaSO coated lime particles are cooled from a preferredtemperature above 850 F. in quenching chamber to ambient temperature thesudden thermal shock gives rise to the beforedescribed cracks 13 whichhave an important functional aspect. The cracks in the CaSO, coatingcover, do not suppress the water reaction of the CaO particles used as aconstruction material and is entirely and definably new. Such a unique,cracked structure has never been used in actual practice, nor evensuggested by any prior practices. The core 35 is substantially pure CaOand the particle size of cracked, calcium sulfate coated particles is ashereinbefore defined. Hence, the covered, unspent CaO still reactsreadily with the fly ash to be discussed later.

The solid product from fluidized lime reactor 28 will containwell-blended, unspent lime and CaSO and is collected in storage bin 50.Each particle of the solid product has only CaO in the core and pureCaSO cracked coating. Since the CaSO coatings are formed on CaOparticles in the reactor which has a very narrow temperature range, i.e.850 F. to 880 F. The reacting gasses in the reactor contain over 90percent SO if the convertor is operated in optimum temperature of 850 F.to 880 F. Hence the reaction is well controlled, and the reactionproduct is simple and pure. The product contains CaO and 02150 and eachcompound coacts with the other and serves its unique and useful functionas construction materials. It is understood that the feed rate mayaffect the percentage of unspent lime in the outflow of solids. Theproperties of the solid can be modified by altering the unspent limepercentage. The solid product can be used:

(A) For making flooring plaster: It is known that large pure CaSOparticles set very very slowly, usually in a number of days or weeks. Infact, hard-burned CaSO can never set by itself (I. W. Mellor, AComprehensive Treatise of Inorganic and Theoretical Chemistry, vol. 3,pp. 774-775, Longmans) When the solid product is mixed with water, thelime in the core areas will leach out and.

serve as an accelerator in CaSO hardening. The initial set takes placein less than 24 hours. In making flooring plaster by hydration, theCaSO, to CaO ratio should generally be about 1:3, though in cases thissame ratio may be reduced to 1:4 or increased to 14:1 in the extreme. Ifwhen the solid product is further calcined to l,000 C. for one hour, theunspent lime and CaSO become sintered and form a very hard flooringplaster upon hydration.

(B) For reclaiming fly ash: The solid materials collected from the dustcollector 14 and precipitator 16 system and those from the lime reactor42 are then blended together in blender 18. The blending ratio maychange the unspent lime concentration and the percentage of 80;; in themixture. The properties of the mixture can be modified by altering theblending ratio in order to make the mixture particularly suitable for aspecific application. The blended mixture of CaSO unspent lime, and flyash may be sold as such, for uses such as in construction materials,soil conditioners, liquid waste treatments, and the like. However, inmany cases, cement, clinker, clay, rocks (in the form of sand, gravel,or chips and fragments) may be added to the blended mixture for themanufacture of brick, cinder blocks or other concrete structure members.In concrete manufacture, the gypsum serves the purpose of regulating thesetting time of cement. If CaSO concentration is too high, there is thedanger of gypsum expansion during hardening because of a quick reactionof CaSO with tricalcium aluminate (in the fly ash) to form alow-density, large-volume compound called calcium sulphoaluminate.Hence, the maximum allowable gypsum concentration has generally beenspecified at about 5% by weight of cement. If anhydrous sulfate is used,it sets with very little expansion.

Fly ash with its silica and alumina readily reacts with lime in thepresence of water to form a compound of cementitious value (Pozzolanicactivity). If the lime particles are covered on their surfaces with thin(IS-micron) cracked CaSO layers, such reactions will quickly expose thenormally unreachable inside of CaSO layers for their actions in theblended mixture. Thus, the CaSO is greatly and rapidly activated in itsentirety, permitting minimum concentration of CaSO to be used in themixture for given results in cement setting control; or resulting incomplete, effective usage of CaSO in the final product.

-In ordinary concrete mixtures, the CaSO (gypsum) is in the form oflarge particles, up to several millimeters in diameter. The regulatingeffect of these CaSO particles is slow and incomplete, being governed bysolid-state diffusion. Further, the gypsum expansion in this particulateCaSO introduces high tensile stress concentration and causes cracks.Hence, the range of CaSO, concentration is often very narrow andcritical, i.e., from 4 to 5% by weight, of cement produced. On the otherhand, the CaSO from my invention has the new, film (about 15 micronsthick) structure covering the unspent lime particles. Thesurface-to-volume ratio in this CaSO is very high, being increased overthe conventional particulate CaSO by a factor estimated to exceed 10.The CaSO reaction is now fast and complete, thereby allowing muchreduced CaSO concentration in the mixture. At the same time, even muchhigher CaSO concentration is tolerable because the high stress build-upis not present. Partially because of the presence of cracks in view ofthe thinness of and presence of cracks in the CaSO surface layer, therange of CaSO in my new concrete mixture can be widened (safely to 2-8%by weight of CaSO -fly ash mixture) and is non-critical.

(C) For making cement: A carefully blended mixture of the solid product,cake, shale, and pyrites is charged to a kiln which is fired withpulverized coal and the end product is cement and exit gasses containinghigh percent of S0 When the solid product, the particles with lime incore area surrounded by cracked CaSO is heated to above 2,000 F. butbelow 2,550 F. in a rotating furnace, S0 and S0 are released, leavingCaO behind:

Additions such as iron pyrites, iron oxide and lead oxide increase thespeed of decomposition and lower the temperature of decomposition. Theexit gas from the furnace, rich in S0 and S0 can be used for H orammonium sulphate production. The regenerated CaO together with theunspent lime from the solid product are recycled to the lime reactor forfurther 80;; removal. By recycling the CaO, materials handling problemwill be greatly minimized.

Dust particles in the gas discharged from the lime reactor are alsoseparated by an electrostatic precipitator 52 or other means. Theeflluent gas will now be free of S0 S0 and dust. To conserve energy, thesame gas is introduced into an air preheater 54 where the temperaturedrops to about 250 F. before discharging as stack gas efiluent. Sincethe effluent gas still has relatively high temperatures, moisturecontained therein will not condense in the stack, and its dispersion inthe atmosphere is swift.

I have thus solved a problem of air pollution, and satisfied along-standing need for a commercially acceptable system for airpollutants disposal. However, the

process is not to be construed as limited to the particular formsdescribed herein, since these are to be regarded as illustrative ratherthan restrictive. For example, engineers can readily use the principleof this invention to design air pollution control equipments for otherpolluting industries such as oil refining, coal coking, metal smelting,ore roasting, and waste incinerating. Further chemicals other than CaOmay be used to stabilize at least one of the S and S0 gasses to formstable, non-gaseous chemical reaction products which can be easilydisposed. For example, magnesia, MgO, closely resembles CaO, is almostinvariably present in commercial lime. It reacts with S0 S0 to form MgSOjust like CaO, but it does not combine with silica or alumina to formcompounds of cementitious value as construction materials. Since theslacking of magnesia is accompanied by an increase in volume andcracking of the hardened concrete, the allowable magnesia contents incement is limited to 5%. In addition, fixed-bed lime reactors can besubstituted and designed to replace the fluidized bed reactorhereinbefore described.

Although the present invention has been illustrated and described inconnection with a few selected example embodiments, it will beunderstood that they are illustrative of the invention and are by nomeans restrictive thereof. It is reasonable to expect that those skilledin this art can make numerous revisions and adaptations of the inventionand it is intended that such revisions and adaptations will be includedwithin the scope of the following claims as equivalents of theinvention.

What 1 claim is:

1. A process for continuously decontaminating flue gas derived generallyfrom burning fossil fuels, comprising the steps of: continuously passinga flow of such gasses through a catalytic reactor to effectsubstantially complete oxidation conversion of the sulfur dioxidecontent of the gasses to sulfur trioxide, continuously bringing thesulfur trioxide containing flue gasses through a fluidized bed of excesscalcium oxide at a temperature above 650 R, such bed being in the formofcalcium oxide particles passing through a No. 8 sieve and which areconfined within a reaction vessel and including an inlet and an outlet,commingling all of the gaseous flow and solid particulate calcium oxideso that substantially all of the sulfur trioxide content is reacted onsurfaces of the calcium oxide to form surrounding layers of calciumsulfate as shells on the calcium oxide particles, monitoring the sulfuroxide content of the gasses to determine the amount of calcium oxidecontent of said fluidized bed, continuously removing the reactionproduct of the sulfur trioxide and calcium oxide, venting thesubstantially sulfur-trioxide-free gaseous flow of flue gasses from thereaction vessel, after exposure to the finely divided particulatecalcium oxide material, and continuously replenishing the fluidized bedwith additional unreacted particulate calcium oxide material.

2. The process in accordance with claim 1 wherein the reaction betweenthe sulfur trioxide and calcium oxide occurs within the reaction vesselat a temperature range of 650 F.950 F.

3. The process in accordance with claim 1 wherein the reaction betweenthe sulfur trioxide and calcium oxide occurs within the reaction vesselat a temperature above 650 F. and, preferably above 850 F.

4. The process in accordance with claim 1 including the steps ofcontinuously monitoring the sulfur oxide content of the gaseous flowprior to entry into said reaction vessel and correlating the amount ofcalcium oxide injected into said vessel with the sulfur oxide content,and continuously removing the solid reaction product of the sulfur oxidegas and calcium oxide.

5. The process in accordance with claim 1 wherein the flow of sulfuroxide gas is passed continuously through the fluidized bed of calciumoxide and the unspent, and reacted calcium oxide particles are removedand blended together.

6. The process in accordance with claim 1, including the step ofquenching the reacted calcium oxide particles after they are removedfrom the reaction vessel.

7. The process in accordance with claim 6 including the step ofcontinuously monitoring the flow of gasses from such vessel to provide acontinuous monitoring of the efiiuent gasses vented to atmosphere.

8. The process in accordance with claim 6 wherein the particle size ofcalcium oxide is proportional to provide a surface/volume ratio of suchsize as to completely react all of the sulfur oxide gas in said reactionVessel.

9. The process in accordance with claim 6 wherein the calcium oxide isprovided in excess quantity within said ,vessel and the spent andunspent calcium oxide particles are blended together.

10. The process in accordance with claim 1 wherein the reaction betweenthe sulfur trioxide and calcium oxide occurs within the reaction vesselat a temperature range of 850 F.-880 F.

References Cited UN [TED STATES PATENTS 2,718,453 9/ 1955 Beckman423-655 3,508,868 4/1970 Kiyoura 23-119 3,411,865 11/1968 Pijpers et al232 2,021,936 11/1935 Johnstone 423242 3,632,306 1/ 1972 Villiers-Fisheret a1. 423-242 FOREIGN PATENTS 435,560 9/1935 Great Britain 23-2 S21,667 2 2/ 1968 Japan l06l03 OSCAR R. VERTIZ, Primary Examiner G. A.HELLER, Assistant Examiner U.S. Cl. X.R.

